Catalyst physical form

Type Silicone emulsion silicone polymer system Products DOW CORNING 1111 Emulsion now CORNING T4-0149 crosslinker DOW CORNING 1R2A or 164 catalyst Physical Form Water dilutable liquids... 

Catalytic properties are dependent on physical form, principally the exposed surface area which is a function of particle size. Industrial PGM catalysts are in the form of finely divided powder, wine, or gauze, or supported on substrates such as carbon or alumina (see Catalysis Catalysts, supported). 

Tubular Fixed-Bed Reactors. Bundles of downflow reactor tubes filled with catalyst and surrounded by heat-transfer media are tubular fixed-bed reactors. Such reactors are used most notably in steam reforming and phthaUc anhydride manufacture. Steam reforming is the reaction of light hydrocarbons, preferably natural gas or naphthas, with steam over a nickel-supported catalyst to form synthesis gas, which is primarily and CO with some CO2 and CH. Additional conversion to the primary products can be obtained by iron oxide-catalyzed water gas shift reactions, but these are carried out ia large-diameter, fixed-bed reactors rather than ia small-diameter tubes (65). The physical arrangement of a multitubular steam reformer ia a box-shaped furnace has been described (1). 

Pesticides. Many pesticides are highly concentrated and are in a physical form requiring further treatment to permit effective appHcation. Typically carriers or diluents are used (see Insectcontroltechnology). Although these materials are usually considered inert, they have a vital bearing on the potency and efficiency of the dust or spray because the carrier may consist of up to 99% of the final formulation. The physical properties of the carrier or diluent are of great importance in the uniform dispersion, the retention of pesticide by the plant, and in the preservation of the toxicity of the pesticide. The carrier must not, for example, serve as a catalyst for any reaction of the pesticide that would alter its potency. 

The gross physical form of a catalyst is chosen to conform to the type of process to be used. The chemical and catalytic characteristics are chosen to achieve the desired reaction and, as an important corollary, to avoid undesired reactions. 

Hydrogen molecules pile up on the surface of the catalyst and form a multimolecular layer in such a way that the bottom molecules are chemisorbed, whereas upper molecules are only physically adsorbed. 

Clearly, there is an infinite number of variations of the physical form of the catalyst which may be employed. One of the major problems is the production of a reproducible metal surface. Irreproducibility may be due to any one of a number of factors, the following being some of the more important ones (a) variations in the degree of cleanliness and state of reduction of the surface (b) variation in the degree of exposure of certain crystallographic planes (c) variation in the concentration of surface defects and (d) variation in the distribution of particle sizes. Most of these factors are not readily controllable and may not be without effect upon the rate and mechanism of the reaction being catalysed it is important, therefore, that the effects of each of the variables is assessed independently. 

FIGURE 2 Some physical forms of heterogeneous catalysts. 1, Particulates 2, extrudates, 3, powders 4, rings 5, monoliths 6, tablets 7, spheres 8, carbon powders and particulates. 

There are cases where it is actually desirable to operate under conditions of mass-transport control this is so for example where it is an intermediate product that is wanted fat-hardening is a case in point. More usually, however, one wishes to work under conditions where conversion is as close as possible to 100% and here it is inevitable that mass-transport control will apply, at least at the end of the catalyst bed. The physical structure of the catalyst then becomes of great importance, and much thought and skill is exercised in maximising access of reactants to the active centres. The form of reactor and the appropriate physical form of the catalyst have to be chosen with care. 

For the overall performance of potential catalysts in practical application additional factors, such as number of active sites, physical form, and porosity must also be taken into account. The classical commercial iron catalyst is an unsupported catalyst. First of all iron is a cheap material and secondly by the incorporation of alumina a surface area similar to that attained in highly dispersed supported catalysts can be obtained. Of course, for an expensive material such as the platinum group metals, the use of a support material is the only viable option. The properties of the supported catalyst will be influenced by several factors [172]... 

Activated carbons are produced with a wide range of properties and physical forms, which leads to their use in numerous applications (Table 1). For example, their high internal surface area and pore volume are pertinent to their being employed as adsorbents, catalysts, or catalyst supports in gas and liquid phase processes for purification and chemical recovery. General information on the manufacture, properties, and applications of conventional activated carbons can be found in Porosity in Carbons, edited by John Patrick [I],... 

The chemical and physical stability of amorphous pharmaceutical materials is controlled by the same basic factors as for crystalline materials [i.e., molecular structure (chemistry), purity (absence of catalysts, chemical reactants, or nucleating agents), molecular orientation (physical form), and molecular mobility (related to temperature)]. For any sample of a given molecular structure and purity, there will be more possible molecular orientations that occur in an amorphous sample than in a crystalline sample. Thus... 

Other mechanisms that could also explain the detrimental interaction between the two catalysts include 1) the solid state reaction between y-alumina and ZnO from the methanol catalyst to form bulk mixed oxides, and 2) the formation of physical agglomerates from the fines of the two catalysts. These are under investigation. 

There is thus a very large number of potentially variable factors. Some of these, such as the chemical nature of the reactant and the metal, are readily controlled others, such as the physical form of the catalyst, are difficult to control and their investigation requires elaborate equipment. With so many variables, one may forgivably be daunted by the magnitude of the total task of assessing the relevance of each variable to the reaction mechanism and of understanding with precision its mode of operation. It is therefore desirable to consider carefully what simplifications are possible, and what is the minimum of information we must have before a knowledge of the reaction mechanism can be claimed. 

The kinetics reported for acetylene hydrogenation are shown in Table XXVIII. Where comparison can be made, they seem to be independent of the physical form of the catalyst and of the support. 

It should be noted that particularly in the case of operations on the commercial scale many other facts besides the nature and physical form of the catalyst itself must be taken into consideration. For example, the progressive loss in activity of a given catalyst, if too rapid, may act as a very serious obstacle to the ultimate success of a commercial process. This is sometimes due to impurities in the reacting substance,—a so-called... 

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